In the specification of Japanese Laying-Open Patent Publication No. 52-118494 (1977), compounds represented by the general formula ##STR1## wherein R.sup.1 is an alkyl group or an aryl group; R.sup.2 is a hydrogen atom, a halogen atom, a nitro group, a carboalkoxy group or an alkyl group; R.sup.3 is an alkyl group or an aralkyl group and Y is a --SO-- or --SO.sub.2 -- group, are disclosed and it is described that these compounds exhibit an activity inhibiting proliferation of cancer cells and are effective as anticancer agents. Also it is described that these compounds are useful agricultural chemicals such as herbicides or as intermediates for synthesizing pharmaceuticals. However, it has not been known that these compounds can be used for control of plant diseases.
We carried out an intensive study in search of compounds which are useful as fungicides and effective for control of phathogenic fungi or genus Alternaria among derivatives of 5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one and we have found that a specific class of 5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one derivatives have a fungicidal activity against Alternaria sooty spot of Chinese mustard (Brassica Rapa var. pervidis) caused by Alternaria brassicicola, apple Alternaria leaf spot caused by Alternaria mali, pear black spot caused by Alternaria kikuchiana, etc. far better than "Captan", copper 8-oxyquinolinate, "Propineb", etc. which have been used for the control of these diseases.